Conventionally, control apparatuses, such as programmable controllers (PLCs), have been employed in factory automation (FA) for controlling mechanical devices. In recent years, such a control apparatus is desired to have a safety function of stopping a mechanical device when an anomaly occurs, to thus ensure safety. Hence, safety control devices equipped with such a safety function have been proposed (see, e.g., JP-A-2002-358106).
In FA, a plurality of control apparatuses are connected by way of communication lines and form a network system, to thus perform simultaneous control of a plurality of mechanical devices. Hence, a communication error which arises between safety control devices must be detected accurately for enhancing the reliability of the safety function.
Meanwhile, among safety control devices forming a network system as described above, communication is carried out with use of frames in accordance with, e.g., the high-level data link control (HDLC) protocol. During the course of communication, a communication error is detected by a parity check or by a cyclic redundancy check (CRC), and corrected.